The suspected gunman at Fort Hood in Texas argued heatedly with fellow soldiers before going on a shooting spree that left three dead and 16 injured at the expansive US Army base, a military investigator said on Friday.
The suspected shooter Ivan Lopez, a 34-year-old soldier battling mental illness, then turned the gun on himself in the second mass shooting at the base in the last five years.
"We do have credible information he was involved in a verbal altercation with soldiers from his unit just prior to him allegedly opening fire," Christopher Grey, of the US Army Criminal Investigation Command, told a news conference, without offering further details.
"At this time, we have not established a concrete motive," Grey added.
Lopez purchased the weapon used in the shooting, a .45 caliber Smith & Wesson handgun, on March 1 in Killeen at Guns Galore, the same shop where former Army psychiatrist Major Nidal Hasan bought the weapon used in the 2009 rampage at the base where he shot dead 13 people and wounded 32 others.
Investigators from the military, Texas Rangers and the Federal Bureau of Investigation have interviewed more than 900 people to gather details of the crime scene that played out over an area covering about two city blocks, Grey said.
The three shooting victims were also identified.
One was Army Sergeant Timothy Owens, 37, of Illinois, who served as a heavy vehicle driver and had been deployed to Iraq and Kuwait.
Another casualty was Staff Sergeant Carlos Lazaney Rodriguez, 38, of Aguadilla, Puerto Rico, who served as a unit supply sergeant with deployments to Kuwait and Iraq.
The third victim was Sergeant First Class Daniel Ferguson, 39, of Florida, who served as a transportation supervisor and had been deployed to Kuwait, Iraq and Afghanistan.
His fiancée and fellow soldier Kristen Haley told Tampa broadcaster WTSP-TV he died while trying to barricade a door to keep the shooter away. She was nearby when the shooting started.
"If he wasn't the one standing there holding those doors closed, that shooter would have been able to get through and shoot everyone else," she told WTSP-TV.
The suspected shooter enlisted in 2008 and had served two tours of duty abroad, including four months in Iraq in 2011, military officials said. He had no direct involvement in combat and had not been wounded.
'Something's gone wrong'
The Lopez family, who live in Guayanilla, Puerto Rico, said in a statement that it is "dismayed" by the tragic events.
"This is a very painful situation," said Ivan Lopez's father in the statement. "I ask for prayers for the affected families."
"My son could not have been in sound mind. He was not that way," he said.
The rampage is the third shooting at a military base in the United States in about six months that, along with a series of shootings in public places like schools and shopping malls, has intensified a national debate over gun violence.
It has also raised questions about security at US military installations, such as Fort Hood, home to some 45,000 soldiers and airmen assigned to the 335-square-mile (870-square-km) base, along with thousands of civilian employees.
"Obviously we have a gap. Anytime we lose an individual, something's gone wrong," US Defense Secretary Chuck Hagel told reporters in Hawaii on Thursday.
Military officials have so far ruled out terrorism as the reason for the attack.